Ultrasonic tissue characterization devices which produce an audible signal which characterizes the biological tissue through which an ultrasonic signal travels are known as shown in U.S. Pat. No. 4,922,917--Dory. There, pulse echo signals from an ultrasonic transducer are used for production of an image of a region of the biological tissue at a visual display. Echo signals from along a scan line in the region are converted to digital signal form and the digital signal is written into memory. Any one of eight segments, or zones, of the memory may be read out at a slow clock rate so as to transpose the ultrasonic frequency of the reflected spectrum into audio frequency signals. The audio frequency signal read from memory is converted to analog signal form and supplied to an electro-acoustic transducer for listening to the same. The 1/8th segment of the scan line of the image display which corresponds to the 1/8th segment converted to an audible signal is displayed with overbrilliance to provide the operator with an indication of the segment of the scan line from which the audible signal is obtained. A disadvantage of such an arrangement is that the operator must switch through 8 segments to listen to a complete scan line of audio information. Also, viewing of the image display, or knowledge of a switch setting, is required to provide the operator with information relating to the depth within the tissue from which the audio signal segment is obtained.
Conversion of an audio frequency signal to stereophonic signal form is shown in U.S. Pat. No. 4,594,731--Lewkowicz. There, an electronic stethoscope is shown which includes a contact phone-transducer for the detection of heart sounds. The heart sound signals are frequency multiplied, and the frequency multiplied signals are supplied to first and second amplitude modulators, or variable time gain amplifiers. The gain of one amplifier is ramped up while that of the other is ramped down during the heart cycle whereby the heart beat appears to travel from the listener's right to the listener's left thereby making it easier to detect when, during the heart cycle, a particular sound is occurring. No assistance in depth perception is provided by the stereophonic movement effect of Lewkowicz.